Because websites are more complex than ever:
With that, there are untold number of subtle errors that can occur on the front-end.
92% of developers believed that TDD yields higher quality code, 79% thought that TDD promotes simpler design and 71% thought the approach was noticeably effective.
80% of the professional developers held that TDD was an effective approach and 78% believed the approach improves programmers' productivitySource: An Initial Investigation of Test Driven Development in Industry
[They] realized a 20.9% decrease in test defects. Additionally, customer-reported defects during the first two years of field use increased by 2.9X while the customer base increased by 10X, indicating a relative decrease in customer-reported defects.Source: On the Effectiveness of Unit Test Automation at Microsoft
According to WHO, there are 285 million people worldwide who, due to some disability (i.e. they are suffering with low vision), cannot read all content on a website. 39 million of those people are blind and cannot access any of the content via sight.
Additionally, there are 360 million people suffering from hearing loss worldwide.Source: WHO Visual Impairment and Blindness Fact Sheet
71% of people with disabilities in the UK use the Web to find information on goods and services.
After CNET started providing transcripts, they noticed a 30% increase in traffic from Google
They doubled their visitor numbers, cut maintenance costs by two thirds, and increased natural search traffic by 50%.Source: W3C Case Studies of Accessibility Improvements
A 1-second improvement correlated to a 27% increase in conversion rate.
Conversions were 27% higher for visitors who enjoyed a load time that was one second faster.
Pages that loaded in 2.4 seconds experienced a 12.8% bounce rate, while those that loaded in 3.3 seconds had a bounce rate of 20%.Source: SOASTA Case Study
46% will not return to a site that performs poorly.Source: SOASTA 23 stats you should know about mobile web performance